X-ray tube target and X-ray tubes utilising such a target

ABSTRACT

The present invention relates to a graphite target for X-ray tubes. In order to avoid the drawbacks of graphite carrier (1) targets due to transformation into carbide of the photo-emissive coating (3) by the carrier material, the invention provides for the formation of the intermediate layer (2) in these targets, in the shape of two sub-layers one of which (20), in contact with the carrier, is made of a refractory material which does not form a carbide (iridium for example), and the other of which (21) is made of another refractory material (tantalum for example), which prevents diffusion of carbon from the carrier into the emissive layer.

D United States Patent 11 1 [111 3,890,521

Shroff 1 June 17, 1975 1 X-RAY TUBE TARGET AND X-RAY TUBES 3.649.355 3/1972 Hennig 313/330 UTILISING SUCH A TARGET 3,731,128 5/1973 Haberrecker 313/330 [75] Inventor: Arvind Shmfi Paris France Primary Examiner-Cameron K. Weiffenbach [73] Assignee: Thomson-CS1", Paris, France Attorney, Agent, or FirmRoland Plottel, Esq.

[22] Filed: Dec. 27, 1972 Appl; No.: 318,904

Foreign Application Priority Data Dec. 31, 1971 France 71.47766 References Cited UNITED STATES PATENTS 3/1970 Clement et al. 117/217 [57] ABSTRACT The present invention relates to a graphite target for X-ray tubes. In order to avoid the drawbacks of graphite carrier (1) targets due to transformation into carbide of the photo-emissive coating (3) by the carrier material, the invention provides for the formation of the intermediate layer (2) in these targets, in the shape of two sub-layers one of which (20), in contact with the carrier, is made of a refractory material which does not form a carbide (iridium for example), and the other of which (21) is made of another refractory material (tantalum for example), which prevents diffusion of carbon from the carrier into the emissive layer.

2 Claims, 1 Drawing Figure X-RAY TUBE TARGET AND X-RAY TUBES UTILISING SUCH A TARGET BACKGROUND OF THE INVENTION a. Field of the Invention The present invention relates to a target for X-ray tubes, more particularly a rotating target. and to the X-ray tubes utilising such a target.

b. Discussion of the Prior Art Targets are known which are constituted by a carrier or base of graphite, to which there is applied an emissive layer producing an X-ray stream under the impact of electrons coming from the tube cathode.

Amongst the various elements of high atomic number which can be used for the emissive layer. tungsten is particularly advantageous and is often given preferencev However, a tungsten layer applied directly to the graphite carrier. at the temperatures encountered during operation. undergoes a chemical reaction with the carbon of the carrier and is quickly transformed to tungsten carbide throughout its thickness; the carbide layer thus formed becomes detached from the carrier.

Solutions which are designed to overcome this drawback. are already known from the prior art.

Thus. it has been proposed that there be arranged be tween the carrier and the tungsten layer. an intermediate tantalum layer which is transformed to a carbide; the layer of tantalum carbide thus formed prevents diffusion of the carbon and thus prevents the formation of tungsten carbide. However. the fragility ofthe tantalum carbide means that the assembly becomes detached from the carrier because of the differences in coeffcient of expansion. Again, it has been proposed that an intermediate layer of rhenium be used. Rhenium does not form a carbide but. with carbon and at 2480C, it forms a eutectic structure containing 16.9 carbon atoms percent. Tungsten carbide forms by contact between the tungsten and this carbide-rich alloy. The rhenium layer is therefore ineffective in preventing the formation of tungsten carbide.

SUMMARY OF THE INVENTION The object of the present invention is to improve targets of the kind hereinbefore described. More precisely, the invention relates to a target structure for X-ray tubes. on a graphite base, which prevents too rapid transformation to carbide on the part of the emissive layer and gives the tube a long surface life.

To this end, the invention provides for the arrangement between the carrier or base, and the emissive layer, of an intermediate layer made up of two sublayers as described hereinafter.

According to the invention there is provided a target for X-ray tubes, constituted by a carrier ofgraphite and an emissive layer made of a refractory metal having a high atomic number. covering said carrier and the two layers being separated by an intermediate layer. characterised in that said intermediate layer is constituted by two sub-layers one of which. that in contact with the carrier and referred to as the first sub-layer. is made of a refractory material which does not form a carbide and is in contact with the carrier, and the other of which. that is the second sub-layer. is in contact with the emissive layer and made of another refractory material.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE illustrates an embodiment of a target in accordance with the invention:

DESCRIPTION OF THE PREFERRED EMBODIMENTS On a graphite carrier 1, there are successively deposited a sub-layer 20 ofa refractory metal which does not form a carbide and then, on this first sub-layer, a second sub-layer 21 of another refractory metal. These two sub-layers constitute the intermediate layer 2. On this latter, there is then deposited the emissive layer of refractory metal, 3.

In a preferred embodiment of the invention, the sublayer 20 is constituted by iridium, the sub-layer 21 by tantalum. and the emissive layer 3 by tungsten.

These different metals are deposited upon the carrier 1 by any known prior art method, for example:

in the case of iridium, by cathode-sputtering to a thickness of around 50 am;

in the case of tantalum, by chemical reaction in the vapour phase, between the chloride and a hydrogen flow at a temperature T at least equal to [000 C and at a pressure P of less than or equal to l torr, this in order to avoid any hydride formation; an example of the conditions employed is given below:

P l torr T 1 C Tun-ls hydrogen flow I00 cm lmn thickness of deposit: 50 am in the case of tungsten, by chemical reaction of the gaseous fluoride with a hydrogen fow, under the following conditions:

P lO' torr T l300C Ratio H /WF 3 thickness of the layer: 500 um During the initial operation of the tube, carbon diffuses across the iridium and reacts with the tantalum, transforming the tantalum to tantalum carbide which will not allow the diffusion of carbon and prevents the formation of tungsten carbide.

The adhesion of the tantalum carbide to the graphite is ensured because of the plasticity of the iridium sublayer which takes up the effect of the difference in coefficient of expansion.

In other variant embodiments, it is possible to use a sub-layer 20, instead of iridium other elements such as osmium and ruthenium, and for the sub-layer 21, instead of tantalum, other elements such as hafnium, niobium or zirconium.

Of course. the invention is not limited to the embodiment described and shown which was given solely by way of example.

What we claim is:

l. A target for X-ray tubes constituted by a carrier of graphite and an emissive layer made of tungsten covering said carrier and being separated by an intermediate layer, said intermediate layer is constituted by two sublayers one ofwhich is in contact the carrier and is made of an element selected from the group consisting essentially of iridium. osmium and ruthenium, and the other in contact with the emissive layer is made of an element selected from the group consisting essentially of hafnium, niobium. tantalum and zirconium.

till

contact with the carrier and is made of an element scluctutl from the group cunsisting Essentially of iridium, osmium, and ruthenium, and the other is in contact with the emissivc layer is mudc of an element selected from the group consisting essentially of hafnium. nio- 

1. A TARGET FOR X-RAY TUBES CONSTITUTED BY A CARRIER OF GRAPHITE AND AN EMISSIVE LAYER MADE OF TUNGSTEN COVERING SAID CARRIER AND BEING SEPARATED BY AN INTERMEDIATE LAYER, SAID INTERMEDIATE LAYER IS CONSTITUTED BY TWO SUB-LAYERS ONE OF WHICH IS IS CONTACT THE CARRIER AND IS MADE OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING ESSENTIALLY OF IRIDIUM OSMIUM AND RUTHENIUM, AND THE OTHER IN CONTACT WITH THE EMISSIVE LAYER IS MADE
 2. An X-ray tube comprising target means for producing X-rays when impacted by electrons, cathode means for generating electrons, said target means including a carrier of graphite and an emissive layer made of tungsten covering said carrier and being separated by an intermediate layer, said intermediate layer is constituted by two sub-layers, one of which is in contact with the carrier and is made of an element selected from the group consisting essentially of iridium, osmium, and ruthenium, and the other is in contact with the emissive layer is made of an element selected from the group consisting essentIally of hafnium, niobium, tantalum, and zirconium. 